115 lines
3.2 KiB
C
115 lines
3.2 KiB
C
/****************************************************************************
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* libs/libc/math/lib_logf.c
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*
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* This file is a part of NuttX:
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*
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* Copyright (C) 2012, 2017-2018 Gregory Nutt. All rights reserved.
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* Ported by: Darcy Gong
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*
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* It derives from the Rhombus OS math library by Nick Johnson which has
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* a compatibile, MIT-style license:
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*
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* Copyright (C) 2009, 2010 Nick Johnson <nickbjohnson4224 at gmail.com>
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*
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* Permission to use, copy, modify, and distribute this software for any
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* purpose with or without fee is hereby granted, provided that the above
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* copyright notice and this permission notice appear in all copies.
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*
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* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
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* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
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* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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*
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****************************************************************************/
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/****************************************************************************
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* Included Files
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****************************************************************************/
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#include <math.h>
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#include <float.h>
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/****************************************************************************
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* Pre-processor Definitions
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****************************************************************************/
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#define FLT_MAX_EXP_X 88.0F
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/* To avoid looping forever in particular corner cases, every LOGF_MAX_ITER
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* the error criteria is relaxed by a factor LOGF_RELAX_MULTIPLIER.
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* todo: might need to adjust the double floating point version too.
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*/
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#define LOGF_MAX_ITER 10
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#define LOGF_RELAX_MULTIPLIER 2
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/****************************************************************************
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* Public Functions
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****************************************************************************/
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/****************************************************************************
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* Name: logf
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****************************************************************************/
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float logf(float x)
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{
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float y;
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float y_old;
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float ney;
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float epsilon;
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int relax_factor;
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int iter;
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y = 0.0F;
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y_old = 1.0F;
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epsilon = FLT_EPSILON;
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iter = 0;
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relax_factor = 1;
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while (y > y_old + epsilon || y < y_old - epsilon)
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{
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y_old = y;
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ney = expf(-y);
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y -= 1.0F - x * ney;
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if (y > FLT_MAX_EXP_X)
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{
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y = FLT_MAX_EXP_X;
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}
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if (y < -FLT_MAX_EXP_X)
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{
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y = -FLT_MAX_EXP_X;
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}
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epsilon = (fabsf(y) > 1.0F) ? fabsf(y) * FLT_EPSILON : FLT_EPSILON;
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if (++iter >= LOGF_MAX_ITER)
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{
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relax_factor *= LOGF_RELAX_MULTIPLIER;
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iter = 0;
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}
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if (relax_factor > 1)
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{
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epsilon *= relax_factor;
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}
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}
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if (y == FLT_MAX_EXP_X)
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{
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return INFINITY;
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}
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if (y == -FLT_MAX_EXP_X)
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{
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return INFINITY;
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}
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return y;
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}
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